Forms of cytosolic NAD-linked glycerol-3-phosphate dehydrogenase present in rapidly growing mouse ascites tumors and in fetal mice are readily distinguishable from forms of the enzyme functioning in adult normal differentiated tissues by gel electrofocusing and heat inactivation studies. It is the purpose of this project 1. to determine whether these forms can be used as markers for the induction of differentiation of neoplastic cells toward expression of gene products characteristic of adult normal tissue; 2. to attempt induction of differentiation of neoplastic cells in culture and in vivo using forms of NAD-linked glycerol-3-phosphate dehydrogenase as markers; 3. to determine whether tum r cells after induction of differentiation express reduced tumorigenicity but retain capacity to generate prophylactic immunity to rechallenge with the original neoplastic cell line; and 4. to define the multiplicity and role of the various forms of NAD-linked glycerol-3-P dehydrogenases observed in rabbit and mouse tissues, with particular emphasis on the oncofetal forms and whether they are characteristic of dividing cells in general, or are more specifically confined to neoplastic cells. Induction of differentiation in vivo is achieved by implanting tumors subcutaneously under conditions limiting growth. Reappearance of adult differentiated enzyme forms coded for in the tumor cell genome is distinguished from host cell infiltration by use of tumors derived from DBA or C57Bl mice with heat-stable G3PDH in adult differentiated tissues being implanted into normal and immun suppressed Balb/c mice with heat-labile G3PDH in adult tissues, and reciprocally by implanting Balb/c tumors into normal and immunosuppressed DBA host. Allogeneic Ehrlich ascites tumor cells with low G3PDH activity are implanted into Balb/c and DBA hosts and source of G3PDH information that appears is determined by heat inactivation. Induction of differentiation in culture employs Ehrlich and L1210 cell lines with DMSO and bis-acetyl-diaminopentane as inducing agents.